It is now thirty years since the first of the organochlorine insecticides, DDT, was introduced to control insect vectors of human diseases, and twenty-eight years since the development of resistance to DDT was suspected in the housefly, Musca domestica, and the common house mosquito, Culex pipiens. During that period resistance has developed to some insecticide group or other in one hundred and eight species of arthropods of public health importance. Although the past six years have seen only six species added to that list, these recent years have been characterized by some dramatic developments. Not only resistant populations have spread over even wider geographical areas but some species become, in the same area, simultaneously resistant to almost all available insecticides while many other species are only susceptible to compounds that many developing countries cannot afford to purchase for their public health programmes.